EVOLUTION OF THE POPULATION OF PHYTOPHTHORA INFESTANS IN FRANCE MEASURED BY EPIDEMIOLOGIC AND PHENOTYPIC MARKERS

Abstract:
The Plant Protection Service has been monitoring mating type and fungicide (metalaxyl) resistance in Phytophthora infestans in France since 1993. Prior to 2003, few A2 strains were detected and a range of metalaxyl resistance levels was present, generally found in less than 40% of the samples.
Since 2003, significant changes have been occurring in the population in northern France, with the presence of A2 strains and metalaxyl resistance steadily increasing, with up to 80% of the isolates resistant in 2007. Metalaxyl resistance increased between 2003 and 2006, but appears to have stabilised in 2007. No resistance to other fungicides has been detected.
Results from other regions of France differ.
In addition to population analyses, the Plant Protection Service provides growers with a warning system and a decision support system (DSS) based on epidemiological models and field scouting.
Observed epidemics seem to be occurring earlier and are becoming more severe.
The primary cause for this is favourable climatic conditions, although increased aggressiveness in the pathogen population cannot be completely excluded.
At present several questions remain regarding changes in the dynamics of epidemics, including those related to the cause of the changes (seed, climate, pathogen aggressiveness) and those related to consequences of the changes (presence of oospores, earlier outbreaks). Concrete answers to these questions are not yet at hand but control strategies are already being modified.